DE68905400T2 - Optical record carrier and method for writing, reading and erasing information in this carrier. - Google Patents

Description

This invention relates to an optical recording medium having a recording medium layer into which information can be written by applying a light beam to selectively change the light absorption spectrum of regions of the recording medium layer.

The invention also relates to a method for recording, reading and erasing information on such a recording medium.

Recently, there has been a tendency that the demand for optical recording media is increasing significantly year by year as a result of improvements in high density and large capacity. Optical recording media can be classified into three types: 'read only', 'write once' and 'rewriteable'.

Information can be written on a 'write once' type medium by a process which essentially consists of projecting a laser beam onto the recording layer to cause melting or decomposition of the spot onto which the beam is projected, thereby creating geometric pits in the areas of the recording layer exposed to the beam.

Information can also be recorded on a write-once type medium by a process consisting essentially of projecting a laser beam onto the recording layer to change the optical properties, such as the refractive index, of the spot onto which the beam is projected, thereby causing crystalline or non-crystalline states to occur in the areas of the recording layer exposed to the beam.

'Writable-repeat' optical recording media include those in which information is recorded by using a magneto-optical effect or a phase-change effect in the recording layer. In the magneto-optical type recording medium, information is recorded as described below. First, a laser beam is projected onto a recording layer consisting of a film magnetized in a direction perpendicular to the layer surface. Then, by applying a magnetic field from an external source, the direction of magnetization of the areas of the film illuminated by the beam is reversed with respect to that of the unexposed areas, where the area illuminated by the beam is heated above the Curie temperature.

In phase-change type recording media, information is recorded as described below. A laser beam is projected onto the recording layer such that the phase of the area of the recording medium illuminated by the beam is converted from an initial non-crystalline state to a crystalline state or vice versa.

It should be noted that the above-mentioned methods for recording information in optical recording media using 'repetitive writing' use a laser beam as a heat source. These media therefore use 'heat mode' recording systems.

Recently, independent of the "heat mode" recording systems, research and development has been carried out on optical recording media using so-called "photon mode" recording systems. As a typical example of using a "photon mode" recording system, An optical recording medium using an organic compound exhibiting the photochromic effect is known. As an example, reference is made to Japanese Patent Publication No. 62-165751 (1987), according to which information is recorded on a multiplex basis in a recording medium having a plurality of monomolecular films containing organic colorants by irradiating with laser beams having different wavelengths or different polarization angles.

The photochromatic effect is an effect in which a certain solid or liquid material changes its color reversibly depending on the incident light. There are a wide variety of organic compounds that exhibit a photochromatic effect. Such compounds include hydrazone, osazone, stilbene, salicylaldehyde, spiropyran, fulgide, azobenzene and derivatives thereof. Of these organic compounds, a typical example of an optical recording medium using a recording layer made of fulgide is given below. By irradiating ultraviolet rays having a wavelength of about 340 nm and visible rays, fulgide reversibly undergoes intramolecular ring closure as shown in Fig. 3(b) from an intramolecular open-ring structure as shown in Fig. 3(a). As a result, as is well known, a reversible change of the absorption spectrum occurs, which is shown in Fig. 4 by the solid line I and the phantom line II.

For example, the absorption spectrum shown by the solid line I in Fig. 4 is initially produced by irradiating visible rays onto fulgide in advance. Recording is carried out by irradiating a recording beam having a wavelength of about 340 nm is irradiated onto the recording layer. The region of the recording layer exposed to the recording beam changes into a state indicated by the absorption spectrum shown by the phantom line II in Fig. 4. As a result, when, for example, this region is subjected to irradiation of a weak beam having a wavelength of about 350 nm, that is, close to the wavelength of the recording beam, a certain difference in absorbance occurs as shown by ΔT in Fig. 4 between the region subjected to the recording beam and the region free from beam illumination. Thus, by detecting this difference in absorbance, a recorded signal can be read.

On the other hand, the absorbance shown by the solid line I in Fig. 4 recovers itself as a result of irradiation of a visible light beam, i.e., having a wavelength of about 500 nm, onto the region previously subjected to the recording beam to erase the recorded signal. As stated above, since the photochromic effect occurs reversibly, the fulgide recording layer can be used for an erasable optical recording medium.

In the above manner, recording information is written in the optical recording medium using a photochromic effect by irradiating laser beams into the recording medium. The spot diameter of beams focused by the optical lens is limited by diffraction effects. At present, the spot diameter of visible or near infrared rays is limited to about 1 µm. When digital recording is performed in which one-bit information is recorded in the beam spot area, all of these conventional optical recording media are only capable of achieving a maximum recording density of about 10⁸ bit/cm². As a result, there are limitations on the recording density that can be achieved with these conventional optical recording media.

From EP-A-0,193,931 an optical recording medium is known in which the recording medium comprises layers with at least two photochromic dyes capable of forming J-aggregates. Before recording, the colorless dyes are first converted into two differently colored J-aggregates by a two-step initialization process carried out for the entire medium, which involves exposure to UV radiation, followed by a heat treatment using IR radiation. Recording can then be carried out by 'bleaching' areas of each layer of the recording medium by exposure to a beam of a wavelength corresponding to the peak of the specific absorption spectrum for the dye in that layer. In this way, in each spot of the recording medium exposed to a beam, one bit of data can be recorded in each dye layer. This achieves a greater recording density. Recorded data can be erased by repeating the initializing heat treatment process to restore the color of the bleached areas of the recording medium. However, this optical recording medium has two disadvantages. First, the initializing/erasing process requires careful heat treatment to form the J-aggregates. Second, the erasing process inherently erases all recorded data; thus, it is not possible to erase recorded data selectively, e.g., only in one of the dye layers.

To record information using a recording system In order to record more densely using photon mode, research and development has recently been directed towards newer optical recording media that utilize a photochemical hole burning effect. However, there are still a variety of technical difficulties that need to be solved before such media can be suitable for practical application.

According to a first aspect of the invention, in an optical recording medium including a recording medium layer (2) in which information can be recorded by applying a light beam to selectively change the light absorption spectrum of regions of the recording medium layer, the recording medium layer contains at least two organic compounds having different sets of photochromic properties to permit recording and erasing of information independently for each compound, and is characterized in that the organic compounds are dispersed in an optically transparent binder, and the photochromic properties of each compound are reversibly changeable by exposure to rays having wavelengths different from those input to any other of the compounds to permit erasing of information independently for each compound.

The recording medium layer may contain at least two kinds of organic compounds such as hydrazone, osazone, stilbene, salicylaldehyde, spiropyran, fulgide, azobenzene and derivatives of the same dispersed in the binder.

The recording medium layer may contain thioindigo and pyrenethioindigo dispersed in the binder.

The binder can be an acrylic polymer, or it can be made of a polyvinyl chloride polymer or an inorganic compound such as water glass.

The optical recording medium may further comprise a substrate made of glass or plastic on which the recording medium layer is formed.

The above substrate may have a geometric pattern, such as grooves, etc., on its surface.

The above-mentioned recording medium may have a reflective film arranged on the opposite surface of the recording medium layer (2) with respect to the substrate (1).

The above recording medium may have protective films that sandwich the recording medium, whereby the recording medium can be protected against high temperature and humidity.

According to a second aspect of the invention there is provided a method for recording, reading and erasing information on an optical recording medium comprising a recording medium layer in which information can be recorded by applying a light beam to selectively change the light absorption spectrum of regions of the recording medium layer, which layer comprises at least two photochromic organic compounds (A, B) dispersed in an optically transparent binder, each compound having optical properties (IA, IIA; IB, IIB) which change upon exposure to rays having wavelengths (λ1, λ2; λ4, λ3) different from those applied to the other compound or compounds to enable recording, reading and erasing of information independently for each compound wherein the optical properties of each compound are reversibly changeable between those for a respective first absorption spectrum (IA, IB) and those for a respective second absorption spectrum (IIA, IIB), which method comprises the following steps:

- an initialization step in which rays are irradiated with wavelengths (λ2, λ3) which essentially correspond to the respective signal peaks of the second absorption spectrum (IIA, IIB) of the components, so that each compound shows its respective first absorption spectrum (IA, IB);

- a recording step in which, depending on information to be recorded, rays are irradiated with wavelengths (λ1, λ4) which substantially correspond to the respective signal peaks of the first absorption spectra (IA, IB) of the compounds, so that in the regions of the recording medium layer exposed to one or more of the rays, compounds (A - Fig. 1(c); B - Fig. 1(d)) associated with these rays return to their respective second absorption spectra (IIA, IIB);

- a reading step in which rays having wavelengths close to those of the rays used in the recording step and lower in intensity are irradiated and recorded information is extracted by detecting the change (ΔT) in the absorption of each ray occurring between recorded and unrecorded areas of the recording medium layer; and

- an erasing step for erasing recorded information by changing the optical properties of one or more of the compounds (A) by irradiating rays with wavelengths (λ2 - Fig. 1(e)) which substantially correspond to the respective signal peaks of the second absorption spectra (IIA) of the one or more compounds.

Fig. 1 and 2 relate to an embodiment of the present invention, wherein:

Fig. 1(a) is a diagram showing the absorption spectrum of an organic compound A in a recording medium of an optical recording medium;

Fig. 1(b) is a diagram showing an absorption spectrum of an organic compound B in a recording medium of an optical recording medium;

Fig. 1(c), (d) and (e) are diagrams showing the total absorption spectra of a recording medium layer containing compounds A and B; and

Fig. 2 is a sectional view of an optical recording medium.

Figs. 3 and 4 relate to a recording medium of a conventional optical recording medium, wherein:

Fig. 3(a) is a structural formula showing a ring-open state of fulgide used in a recording medium layer of a conventional optical recording medium;

Fig. 3(b) is a structural formula showing a ring-closed state of fulgide used in a recording medium layer of a conventional optical recording medium; and

Fig. 4 is a diagram illustrating a change in the absorption spectrum produced by fulgide depending on the conditions shown in Figs. 3(a) and (b).

Referring to Figs. 1 and 2, an embodiment of an optical recording medium according to the invention will now be described, and the method of using the same will now be described and explained.

As shown in Fig. 2, an optical recording medium comprises a substrate 1 made of glass or plastic and a recording medium 2 formed as a layer on the substrate 1. The recording medium 2 has a composition in which at least two kinds of a variety of organic compounds having different sets of photochromic properties are dispersed in an optically transparent binder. Suitable kinds of organic compounds include hydrazone, osazone, stilbene, salicylaldehyde, spiropyran, fulgide, azobenzene and derivatives thereof.

The binder can hold at least two kinds of such organic compounds in a dispersed state, and accordingly such a binder is suitable for use. For example, an acrylic polymer or a polyvinyl chloride polymer or an inorganic material such as water glass can be used as the binder.

The following description concerns an example in which the recording medium layer 2 contains two kinds of organic compounds A and B, each of which has a photochromic effect in the state dispersed in the binder.

The example will be described with reference to the following conditions. Namely, as shown in Fig. 1(a), the organic compound A exhibits a photochromatic effect in which the absorption spectrum thereof changes reversibly between the state indicated by the solid line i and another state indicated by the phantom line ii. On the other hand, as shown in Fig. 1(b), the organic compound B, when exposed to rays having wavelengths λ3 and λ4, exhibits a photochromic effect in which the absorption spectrum thereof changes reversibly between the states indicated by the solid line i and the phantom line ii. The embodiment uses such organic compounds A and B each having the recording and erasing wavelengths λ1 to λ4 which are used to record or erase information by the organic compounds A and B in the recording layer. The wavelengths λ1 to λ4 have the mutual relationship λ1 < λ2 < λ3 < λ4.

In order to record information in the recording medium, the recording medium is first exposed to the successive irradiation of rays having wavelengths λ1 and λ3 in an initial stage, and then the absorption spectrum indicated by the solid lines IA and IB in Fig. 1(c) is preserved in the recording medium.

Then, when a recording beam having a wavelength λ1 is irradiated onto the recording medium, only the organic compound A in the recording medium undergoes a photochemical reaction, so that the absorption spectrum of the recording medium changes to that shown by the phantom line IIA and the solid line IB in Fig. 1(c). When a beam of lower intensity than that of the recording beam and having a wavelength close to λ1 is irradiated in the above state, a Absorbance difference ΔT between the absorption spectra and after recording with the recording beam having the wavelength λ1, whereby information recorded using the wavelength λ1 can be detected.

Subsequently, when a recording beam having a wavelength λ4 is irradiated onto the recording medium in the state where the inorganic compound A has the absorption spectrum shown by the phantom line II in Fig. 1(c), only the organic compound B undergoes a photochemical reaction, so that the absorption spectrum of the recording medium changes to that shown by the phantom lines IIA and IIB in Fig. 1(d). When a beam having an intensity lower than that of the recording beam and having a wavelength close to λ4 is irradiated in the above state, an absorbance difference occurs between the absorption spectra before and after recording with the recording beam having the wavelength λ4, whereby information recorded using the wavelength λ4 can be detected. In this way, recordings at wavelengths λ1 and λ4 are made independently of each other without affecting the content of any previous recordings made at the other wavelength.

For example, in the state where the organic compound A has the absorption spectrum shown by the phantom line IIA in Fig. 1(d), when a beam having the wavelength λ2 is irradiated onto the recording medium to erase information recorded by the recording beam having the wavelength λ1, the absorption spectrum of the recording medium changes to the state shown by the lines IA and IIB of Fig. 1(e). In this way, regardless of previous recording, made by using other wavelengths, additional recording can be carried out using the wavelengths λ1 and λ4, respectively, independently. Moreover, erasure of recorded information can be carried out by using respective wavelengths λ1 and λ4, respectively, without adversely affecting the content of information recorded by using other wavelengths. Accordingly, the optical recording medium of the present invention provides an information recording capacity doubled as compared with that of a conventional optical recording medium using a single organic compound.

As a typical embodiment of the combination of organic compounds A and B, it is proposed to use thioindigo (having a wavelength λ1 of about 490 nm and a wavelength λ2 of about 540 nm) and pyrenethioindigo (having a wavelength λ3 of about 580 nm and a wavelength λ4 of about 720 nm). However, a wide variety of other combinations of organic compounds A and B can also be used, which have different sets of photochromic properties which then change reversibly when exposed to rays of different wavelengths.

The above embodiment uses two kinds of organic compounds dispersed in the binder. However, it should be noted that the invention also allows the combination of three or more kinds of organic compounds, provided that multiple recording can be carried out based on the principles described above.

Due to the above features, which use a photochromatic effect, recording, reading and erasing of information can be carried out independently with respect to individual organic compounds dispersed in the recording medium. As a result, the optical recording medium according to the invention promotes the information recording density significantly proportional to the number of organic compounds in the recording layer.

Having thus described the invention, it is obvious that it may be modified in many ways.

Claims (11)

1. An optical recording medium comprising a recording layer (2) for recording information by applying a light beam, the light absorption spectrum of regions of the recording medium layer being selectively changed, which layer contains at least two organic compounds having different sets of photochromic properties to enable the recording and erasing of information independently for each compound, characterized in that the organic compounds are dispersed in an optically transparent binder, and that the photochromic properties of each compound are reversibly changeable by exposure to rays having wavelengths different from those introduced into any other of the compounds to effect the erasure of information independently for each compound. 2. The optical recording medium according to claim 1, wherein at least two kinds of organic compounds such as hydrazone, osazone, stilbene, salicylaldehyde, spiropyran, fulgide, azobenzene and derivatives thereof are dispersed in the binder. 3. The optical recording medium according to claim 1, wherein thioindigo and pyrenethioindigo are dispersed in the binder. 4. Optical recording medium according to one of claims 1 to 3, wherein the binder consists of an acrylic polymer or a polyvinyl chloride polymer or an inorganic compound, such as water glass. 5. An optical recording medium according to any one of claims 1 to 4, wherein the absorption spectrum of each of the compounds changes reversibly upon exposure to a beam having a wavelength that does not affect the absorption spectrum of any other of the compounds. 6. An optical recording medium according to any one of claims 1 to 5, wherein the color of each of the compounds changes reversibly upon exposure to a beam having a wavelength that does not affect the color of any other of the compounds. 7. Optical recording medium according to one of the preceding claims, further comprising a substrate (1) made of glass or plastic on which the recording medium layer (2) is formed. 8. An optical recording medium according to claim 7, wherein the substrate (1) is patterned to have a geometric pattern, such as grooves, on one of its surfaces. 9. An optical recording medium according to claim 7 or claim 8, further comprising a reflection film formed on the opposite surface of the recording medium layer (2) with respect to the substrate (1). 10. Optical recording medium according to one of claims 7 to 9, wherein the recording medium layer (2) is arranged between protective films. 11. A method for recording, reading and erasing information on an optical recording medium having a recording medium layer in which information can be recorded by applying a light beam to determine the light absorption spectrum of regions of the recording medium layer to selectively change which layer comprises at least two photochromic organic compounds (A, B) dispersed in an optically transparent binder, each compound having optical properties (IA, IIA; IB, IIB) which change upon exposure to rays having wavelengths (λ1, λ2; λ4, λ3) different from those applied to the other compound or compounds to allow the recording, reading and erasing of information independently for each compound, the optical properties of each compound being reversibly changeable between those for a respective first absorption spectrum (IA, IB) and those for a respective second absorption spectrum (IIA, IIB), which method comprises the following steps: - an initialization step in which rays are irradiated with wavelengths (λ2, λ3) which essentially correspond to the respective signal peaks of the second absorption spectrum (IIA, IIB) of the components, so that each compound shows its respective first absorption spectrum (IA, IB); - a recording step in which, depending on information to be recorded, rays are irradiated with wavelengths (λ1, λ4) which substantially correspond to the respective signal peaks of the first absorption spectra (IA, IB) of the compounds, so that in the areas of the recording medium layer exposed to one or more of the rays, compounds (A - Fig. 1(c); B - Fig. 1(d)) associated with these rays return to their respective second absorption spectra (IIA, IIB); - a reading step of irradiating beams having wavelengths close to those of the beams used in the recording step and lower in intensity, and extracting recorded information by detecting the change (ΔT) in the absorption of each beam occurring between regions of and without recording the recording medium layer; and - an erasing step for erasing recorded information by changing the optical properties of one or more of the compounds (A) by irradiating rays with wavelengths (λ2 - Fig. 1(e)) which substantially correspond to the respective signal peaks of the second absorption spectra (IIA) of the one or more compounds.